An active matrix organic light emitting diode (hereinafter referred to as AMOLED) display panel utilizes OLED to emit lights with different luminance, so that pixel display corresponding to OLED has corresponding luminance. Compared with a conventional thin film transistor liquid crystal display panel (TFT LCD), AMOLED display panel has a faster response speed, a higher contrast and a broader angle of view, and is an important development direction of the display panel.
Current that drives OLED to emit light can be represented by the following equation:
      I    OLED    =            β      2        ⁢                  (                  Vgs          -          Vth                )            2      where Vgs is a voltage difference between a gate and a source of a driving transistor, β is a parameter related to process parameter and characteristic size of the driving transistor, and Vth is a threshold voltage of the driving transistor.
According to the above equation, the driving current that drives a light-emitting device OLED to emit light is related to the threshold voltage Vth of the driving transistor. In the actual application, the threshold voltage Vth of the driving transistor would change in the light-emitting phase, which would affect light-emitting luminance of the light-emitting device OLED, such that the luminance is non-uniform in the process of light emitting, and then the display effect of the AMOLED display panel would be affected badly.